Recently, there have been developed simple assay reagents or kits wherein various measuring items including the infection with pathogens such as viruses and bacteria, the presence of pregnancy and blood-sugar level can be detected or quantitated in a short time of from a few minutes to several tens minutes using antigen-antibody reactions, enzyme reactions and the like. Proteins composing pathogens, human chorionic gonadotropin (hCG), blood-sugar and the like are the subjects of the detection or quantitation. Most of the simple assay reagents have the characteristics that they do not require any special equipment, their operations are also easy and they are low in price. For example, a simple assay reagent for pregnancy diagnosis is sold in general drugstores under OTC. The simple assay reagents measuring the infection with pathogens are widely used also in general hospitals and clinics in addition to large hospitals and medical assay centers unlike other assay reagents. These places are often medical institutions where patients visit first. If the presence of infections in specimens collected from patients could be clarified there and then, therapeutic treatment can be applied in early stages of symptoms. The importance of such simple assay reagents or kits in medical field, therefore, is increasing more and more.
Presently, membrane assay methods, specifically assay methods using membrane such as film or filter of nitrocellulose and the like as simple test methods have been generally known, being roughly classified into flow-through type and lateral-flow type membrane assay methods. The former is a method wherein a solution containing a substance or an analyte to be examined is passed vertically through a membrane coated with a substance for detecting said analyte and the latter is a method developed in lateral direction. In both methods, they have a common point that the detection or quantitation of an analyte in a sample is performed by forming a complex of a membrane-immobilized substance which specifically binds to the analyte, the analyte and a labeled substance which specifically binds to the analyte on a film, and then by detecting or quantitating said labeled substance.
However, when a specimen practically collected from a patient is analyzed by such simple membrane assay method using membranes or filters, so-called “false positive” or “false positivity”, which is a positive judgment made in spite of the absence of an analyte in the specimen, may occur. The occurrence of false positive reaction during measuring the infection with pathogens leads to mistaken information on the patient. Therefore, not only the specification of the cause may be delayed, but also an inappropriate treatment may be taken to lead to serious consequence such as aggravation of the condition of a disease to more severe. Accordingly, suppression of false positive reaction is an extremely important issue in view of the major purpose of use of a simple test method.
Until now, for a so-called immunoassay, a method diluting the sample with a buffer containing a surfactant (for example, JP-A-9-501494), a method absorbing by absorbent filter containing a surfactant (for example, JP-A-2000-502452) and the like have been reported, but they have not been sufficient (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).